Research activities in oncology are increasingly directed to identify novel treatments with enhanced specificity against tumors and directed against precise molecular targets (targeted therapy). Indeed very frequently in tumors it is observed that uncontrolled cell growth is due to altered processes of signal transduction from the cell surface that induce cell proliferation, inhibit apoptosis and which frequently involve overexpression and activation of specific surface receptors. The EGFR family (otherwise called ErbB or HER) is constituted by four transmembrane proteins (EGFR/HER1, HER2, HER3 and HER4) which play multiple roles both in normal cells and in the development and maintenance of tumors [1]. Three aberrant mechanisms contribute to the tumorigenic activity of ErbB receptors: receptor overexpression, often linked to gene amplification; constitutive activation due to specific mutation, ligand overexpression. Given the key role of these receptors in oncogenic signalling, new drugs have been developed, both monoclonal antibodies and small molecules, having as targets in particular EGFR and HER2, and which are widely employed in clinical protocols for the therapy of a variety of tumors, mainly lung, colon and breast. Among these, antibodies like cetuximab, panitumumab (against EGFR), trastuzumab and pertuzumab (against HER2) and tyrosine-kinase inhibitors (TKIs) gefitinib, erlotinib (against EGFR) and lapatinib (against EGFR/HER2) [1]. However, in spite of these progresses, the clinical efficacy of these agents is lower than expected and often is accompanied by the emergence of resistance. For example objective responses observed with trastuzumab (HERCEPTIN™) in patients with HER2 positive mammary tumors are low (in general 15%) and short lived [2]. Furthermore, several prototype tumor vaccines against HER2 based mainly upon the use of peptides and proteins have been developed over the last years and tested in Phase I and II clinical trials, without significant results [3,4]. These vaccines had been designed to induce mainly a cell-mediated immune response, with a principal involvement of cytotoxic CD8+ cells (CTLs). The generation of CTL correlated with the prevention and eradication of HER2+ tumor cells in preclinical models, but was unable to control the diffusion of metastasis in human patients. Subsequently, some studies have demonstrated that tumor cells treated simultaneously with trastuzumab and with CTLs derived from patients vaccinated with peptides were lysed more efficiently [5], thus suggesting that an antitumor vaccine against HER2 capable of inducing simultaneously both an antibody response and a cytotoxic response should be able to achieve a significant enhancement of therapeutic efficacy. More recently, genetic vaccines against HER2 based upon plasmid DNA electroporation into muscle tissue and the use of recombinant Adenoviral vectors have been shown to be able to achieve these goals and have provided promising results [6-8], being able to induce at the same time the development of innate immunity, cell-mediated immunity and, most importantly, high titer antibody responses against the receptor.
A distinctive feature of the members of the ErbB receptor family is the interdependence and complementarity of their functions. While HER2 is a known oncogene and its overexpression, mainly linked to gene amplification in approximately 25% of breast tumors, has been causally correlated to tumor development [9], for HER3 no mutations have been found to be directly involved in the process of carcinogenesis. However, loss of HER3 expression abolishes the transforming ability of HER2. Hence, HER3 can be considered as an obligate partner of HER2-mediated transformation [1,10]. Furthermore HER3 seems to play a key role in the development of resistance to current EGFR and HER2 inhibitors, most likely as a consequence of its overexpression and increased plasma membrane localization: its role appears to be linked to the formation of heterodimers with EGFR and with HER2 and its ability to be transphosphorylated in six tyrosine residues that serve as binding sites for molecules involved in the downstream signalling, such as p85, the regulatory subunit of PI3K [11]. Also cMet amplification has recently been described in cells resistant to TKIs, and under this circumstance, the mechanism of resistance seems to be mediated by HER3 transphosphorylation by overexpressed cMet [12]. The PI3K/Akt pathway is critical for the viability and maintenance of cancer stem cells in breast [13], prostate [14], lung [15], colon [16], brain [17] cancers as well as in leukemias [18]. Given the central role of PI3K in the signalling in cancer stem cells and the inability of HER2 to activate the PI3K axis in the absence of HER3, it can be hypothesized that HER3 plays a fundamental role in cancer stem/progenitor cells. Hence, its inhibition may be a powerful strategy to eradicate these cells and improve efficacy of current therapies.
HER3 consists of an extracellular domain which binds to the ligand (ECD), a dimerization domain within the ECD, a transmembrane domain (TM) and a C-terminal domain (ICD), which is phosphorylated. Neuregulin (NRG) or other ligands bind the ECD and trigger signal transduction promoting receptor dimerization with other RTKs and ICD transphosphorylation. Because HER3 does not possess tyrosine kinase activity, its function can only be inhibited by specific monoclonal antibodies. In literature, there are already evidences that antibodies directed against HER3 can display antitumor activity. Schoeberl and coworkers [19,20] have recently shown in primary tumors and in cell lines that express members of the ErbB family and relevant ligands, hence with autocrine loops, that only anti-HER3 antibodies but not antibodies against EGFR (cetuximab), or HER2 (trastuzumab and pertuzumab), are capable to fully inhibit receptor activation induced by all ligands of this receptor family, whereas cetuximab and trastuzumab are able to neutralize only a subset of them.